Historically, improvements to polymer properties have focused on static properties including strength, thermostability, toughness, and durability. Recent research has broadened to incorporate multifunctionality into polymers that can adapt to their environment, with dynamic properties such as recyclability, remoldability, self-healing, and shape memory. The ability to rework and remold certain polymers is of great interest due to increased awareness of recycling, high cost of materials, and the ability to extend the life of a material through in situ repair. In order to accomplish many of these properties, it is necessary to form and break bonds or crosslinks.
There are two classes of polymer materials that are categorized based on their network structure: thermosets and thermoplastics. Thermosets are polymers that are heated, molded, and cured to form a permanent shape and can no longer be reworked due to network constraints when cured. Thermoplastics will soften upon heating, becoming flowable, which allows them to be remolded multiple times without loss of properties. The biggest distinction between the two classes is the crosslinking architecture of the network. Thermosets tend to have a high density of crosslinking—specifically, interchain covalent bonds—that hold all the polymer chains together. By contrast, thermoplastics are made up primarily of long individual polymer chains that associate together to form a network. This architecture gives thermosets high thermal stability, high rigidity, dimensional stability, and resistance to deformation, which makes them desirable, but removes the ability to recycle, reshape, and repair. Remolding and reshaping thermoplastics is made possible due to non-covalent associations between the polymer chains; however, the reduced number of crosslinks makes the material more vulnerable to creep under stress.
Introducing reversible crosslinking into a polymer network can controllably capture the advantages of both a thermoset and thermoplastic. A conventional problem with crosslinking a network is that the material becomes non-recyclable and non-formable. However, if these crosslinks could be removed, the desirable properties of a thermoplastic-remolding and shaping—are made possible.
Research in reversible crosslinking within materials dates back to the 1960s, where the majority of the work was based on thermally triggered Diels-Alder chemistry. Uniform heating of a part is required, but with Diels-Alder chemistry, uniform heating is generally difficult to achieve for large parts. Moreover, the response is slow and gradual, due to the low thermal conductivity of polymers as well as the bond breakage that occurs over a wide temperature range. Also, structural parts made from this chemistry suffer from heat sensitivity.
New approaches are needed that provide greater flexibility in trigger mechanisms and strategies for crosslinking reversibility. Polymer materials, in addition to being thermoresponsive, can be synthesized to be responsive to chemicals, mechanics, light, and/or electricity.
In chemoresponsive materials, supramolecular-based polymer systems can be altered by components that compete with existing covalent bonds. These systems are difficult and expensive to synthesize, and no clear path can be seen for altering manufactured parts. Polymer materials instead can be synthesized to be responsive to mechanics. Mechanoresponsive materials rely on ultrasonication or other means of mechanical stress to break crosslinking chains. Large material parts would suffer from energy transfer challenges. Reversibility of such systems relies on the re-equilibration of the components with time, a slow and undesirable process. Polymer materials instead can be synthesized to be responsive to electricity. Electroresponsive materials are triggered by an electrical potential that often oxidizes or reduces components.
In view of the shortcomings in the art, what is desired is non-thermal reversible crosslinking in polymers for mechanical tunability, preferably exploiting naturally occurring stimuli, such as natural light. These polymers would be useful for many applications, including (but not limited to) coatings and polymer parts.